Examples of an image forming apparatus of the type having a plurality of functional modules each adapted to achieve particular functions proper to the module include a digital copier and an electrophotographic image duplicating machine. An electrophotographic image duplicating machine, for example, has functional modules including a control panel and a printed output discharge module (hereinafter referred to as page sorter module) in addition to an image reproducing module which is responsible for the reproduction of duplicate images.
Each of the functional modules thus included in such a type of image forming apparatus operates under the control of a subsidiary control unit exclusively associated with the particular functional module. The respective subsidiary control units associated with the individual functional modules in turn are under the control of a single master control unit which is operative not only to control each of the subsidiary control units but to coordinate the operation to be performed by two or more of the functional modules.
For exchange data between the master control unit and each of the subsidiary control units in this type of image forming apparatus, there may be used a data communication network of either the leased transmission line type or the common interface bus type in a control system for the image forming apparatus. In a control system using a communication network of the leased transmission line type, each of the subsidiary control units respectively associated with the different functional modules is electrically coupled to the master control unit through transmission lines proper to the particular subsidiary control unit so that each of the subsidiary control units is permitted to communicate with the master control unit independently of the other subsidiary control units. A communication network of this type however has a drawback encountered when an additional functional module is to be coupled to the apparatus at the user's option. To enable the master control unit of the apparatus to be in control of the additional functional module, additional subsidiary control unit data transmission lines proper to the subsidiary control unit for the additional functional module must be provided and connected to the master control unit.
A drawback of this nature is eliminated in a control system using a communication network of the common bus type in which the respective subsidiary control units associated with the different functional modules are coupled to the master control unit through an interface bus common to the individual subsidiary control units. A particular address is assigned to each of the plurality of subsidiary control units so that the individual subsidiary control units are one after another accessed by the master control unit with the addresses for the subsidiary control units designated successively. A known example of a communication network of this common bus type is taught in Japanese Provisional Patent Publication (Kokai) No. 59-127133.
Each of the main and subsidiary control units of an control system of the described character is typically implemented by a semiconductor microprocessor and is, thus, not only the master control unit but each of the subsidiary control units is capable of executing various functions. This means that each of the subsidiary control units may perform its functions or at least preselected ones of its functions at its own disposal, viz., independently of the master control unit.